The mechanism of venting in an easy-open container end is per se known, for example as described in U.S. Pat. Nos. 4,015,744 of 5 Apr. 1977 and 4,030,631 of 21 Jun. 1977 which are assigned to the assignee of this application. The easy-open mechanism includes an end panel which is attached to the container, a tear panel defined in the end panel by a score line, an operating tab including a nose which extends partially over the tear panel, and an integral rivet connecting the tab to the end panel and defining a pivot point for tab operation. Basically, initial lifting of the tab produces an upward force on the rivet and a downward force on an edge of the tear panel, preferably adjacent a beginning point of the tear panel score line. This in turn causes an initial opening of the tear panel beneath the nose of the tab, a limited expansion of that opening along a vent region, and then further lifting motion of the tab causes the tear panel to separate progressively along its score line, leaving a small integral connection between the end panel and tear panel, about which the tear panel is rotated inward of the container to form an opening through which container contents can be removed.
Particularly in canned carbonated beverages there is substantial internal pressure, e.g. in the order of 100 psi., in the small space between the top of the liquid contents and the product side of the can end. This pressure must vent quickly through a small vent opening in the vent region during the initial opening motion. At present it is customary to define this small vent opening with a hook formation at the radially inner end of the score line and a score stop (also called a check stop or arrestor stop) in the region where the tear panel score line proceeds outward of the end. The score stop is in the form of a shallower (less deep) segment of the score line, of relatively short length, such as described for example in U.S. Pat. No. 4,503,989 issued 12 Mar. 1985. This score stop provides an increase in tear resistance along the score line. Thus, the vent region is defined by the upper hook on one side of the tab, and the score stop on the opposite side.
If the function of the score stop is not included, the releasing high pressure from the can may under some circumstances force rapid continuing separation along the score line, and literally blow the tear panel off the end with considerable force.
There has been a continuing effort (induced by desired economies in metal usage) to use thinner metal for can ends. Until recently, end stock (principally aluminum) was in the order of 0.011 inch or greater in thickness. Presently, ends are being made with metal in the order of 0.009-0.008 inch in thickness. As thinner metal is employed in making can ends, the effectiveness of the score stop has decreased. There simply is not sufficient metal in the thicker stop portion of the score line to resist effectively the continuing score line separation immediately following venting. Therefore, a need exists for another way to retard the score line separation for the initial venting or "pop" when opening of the end commences.
Related to the venting provisions, a so-called "dimple down" bead has been formed in tear panels by simply depressing a section of the tear panel, usually near the vent region of the tear panel score line, inwardly of the end. Normally no coining is performed on this prior art bead, the metal is simply displaced to define the bead, which is intended to add strength to the tear panel during opening, and which also may function when compressed during scoring to insure good integrity in the rivet area.